Wire cut electric discharge machines generally comprise upper and lower wire guides which are respectively disposed above and under a worktable for fixing a workpiece and are individually provided with a wire passage for guiding a wire electrode (hereinafter merely referred to as wire). These machines are arranged to produce an electric discharge between the wire and the workpiece, while running the wire previously inserted through the wire passages and a machining initial hole formed through the workpiece, to thereby carry out electric discharge machining, and are further arranged to inject a machining fluid from upper and lower nozzles, respectively provided at the upper and lower wire guides, toward an electric discharge machining region. When a disconnection of the wire occurs during the electric discharge machining or when the wire is intentionally cut upon completion of the machining for each product during the manufacture of a plurality of products from one workpiece, the electric discharge machines operate to extend the thus accidentally or intentionally disconnected wire between the two wire guides, by causing the wire to be fed through the upper wire guide while restraining the wire by the machining fluid jetted from a wire extension nozzle attached to the upper nozzle, thereby causing the wire to be inserted into the initial hole of the workpiece and then into the wire passage of the lower wire guide.
In case that such automatic wire extension is carried out successfully, the wire does not touch the workpiece during the wire extension until it reaches the lower wire guide. On the other hand, when the wire insertion to the initial hole or to the lower guide is blocked and thus the wire extension ends in failure for the reason, for instance, that a burr exists in the inner peripheral surface of the initial hole or sludge is accumulated in the wire passage of the lower wire guide, the wire is flexed and touches the workpiece. In this respect, conventionally, the automatic wire extension is performed with a predetermined voltage applied to the wire, and this wire voltage is monitored, so as to determine an occurrence of a failure of the wire extension when the wire touches the workpiece which is grounded, namely, a short circuit occurs between the two, and accordingly the wire voltage is dropped to the ground potential. When such a wire extension failure occurs, a similar wire extension operation is repeatedly carried out, and if the wire extension still fails despite a predetermined number of times of wire extension, the wire is extended through another initial hole.
In case that the wire extension is performed through an initial hole with a relatively large diameter suitable for manufacture of ordinary products, the wire extension can be achieved at a practically satisfactory rate. On the other hand, when the wire extension is effected through a small-diameter initial hole suitable for manufacture of products requiring extremely precise machining, such as lead frames of integrated circuits, the chance of succeeding in the wire extension is lowered. In view of this, the applicants hereof proposed a novel wire extension method (Japanese Patent Application No. 63-252893), wherein when carrying out a normal wire extension through an initial hole having a normal diameter, the wire electrode is fed while the machining fluid is injected to the workpiece from a main nozzle positioned at a predetermined distance from the workpiece, and when carrying out a specific wire extension through a small-diameter initial hole, a subnozzle having a nozzle hole diameter smaller than the small diameter of the initial hole is attached to the main nozzle, and the wire electrode is fed with the subnozzle positioned closer to the workpiece and with the injection of the machining fluid interrupted. According to the wire extension method of this type, the wire is more likely to touch the workpiece when inserted through the machining initial hole, and therefore, a wire extension failure cannot be accurately detected by the aforementioned type of wire extension failure detecting method in which the presence/absence of short circuit between the wire and the workpiece is monitored. As a result, the retry function of the electric discharge machine to retry the wire extension upon occurrence of a wire extension failure cannot be effectively used.